Providing README.rst in scripts and pywikibot.

This includes description of existing libraries and scripts in core.
Since many scripts which are part of compat and needs migration their
details are also mentioned with #at their beginning.
Besides, the README.rst in pywikibot-core is updated.

Signed-off-by: xqt <info@gno.de>

Bug:T66847
Change-Id: I3e1a36826167f92502451593565fa719af03ae5e